This invention relates to a digital broadcast system, or in particular to a sending apparatus and a receiving apparatus thereof.
In recent years, the digitization of the television broadcast has advanced. The CS, BS and the terrestrial digital broadcasts have already started for fixed receivers. In the future, the digital broadcast service for mobile units is expected to be started.
In receiving the broadcast by mobile units, as compared with the fixed receivers, the receiving conditions undergo great variations due to such factors as the shadowing, fading, and Doppler shift of the frequency of the radio wave occurring during the movement. An attempt by mobile units to receive images and voices of a quality as high as the broadcast to fixed receivers, therefore, may fail while passing through an environment of a deteriorated field strength. In the receiving operation by mobile units, therefore, it is important to prevent the broadcast service from being interrupted by taking measures against the variations of the radio wave receiving conditions during the movement.
In the digital broadcast, the video information are digitized and sent out in a bit stream. Further, this bit stream is multiplexed with those of other information (the voice, channel information and program information, for example), and after RF modulation, broadcast as a broadcast wave from an antenna. These bit streams are hereinafter referred to simply as streams. Also, if necessary, an image converted to a bit stream is referred to as a video stream, and the voice converted to a bit stream as an audio stream.
The digital broadcast includes a scheme called the hierarchical transmission. In the hierarchical transmission, the band in the broadcast radio channel is divided, and for each band thus divided, a different transmission scheme is used. In the band called a weak hierarchical level, which is narrow in dynamic range that can be secured and not adapted for the receiving operation of a mobile unit, a high-quality image can be transmitted at a high bit rate using a broad band. In the band called a strong hierarchical level, on the other hand, though the image is low in quality with a low bit rate, the transmission is possible by a transmission scheme suitable for mobile receivers.
With the screen size of about 2 or 3 inches of a portable terminal, the viewers are not embarrassed, due to the small screen size, by an image of a quality that can be transmitted at the weak hierarchical level. With the screen not smaller than 7 inches in size mounted on an on-vehicle terminal, however, the viewers desire the image of a quality of at least analog SD (standard) TV or a higher-quality HD (high definition) TV.
The conventional broadcast system (called the simultaneous broadcast) is known in which at the transmitting end, as disclosed in JP-A-2003-143503, a high-quality and low-quality video streams are generated from the same content and sent out at the weak and strong hierarchical levels, respectively. At the receiving end, on the other hand, the high-quality video stream of weak hierarchical level is displayed when the radio wave receiving condition is good condition, while the mode is switched to the low-quality video stream of strong hierarchical level when the radio wave receiving condition is deteriorated. In this way, a high-quality image is displayed in manner not to freeze the image against the variations of the radio wave condition.